1. Field of the Invention
The present invention relates to an improvement of the light-source determining device of a camera for determining the kind of light source, such as an incandescent lamp, a fluorescent lamp or natural light, during photography.
2. Description of the Related Art
It has conventionally been common practice to incorporate a colorimetric sensor into a video camera (still movie camera) for the purpose of adjusting white balance. In the field of silver-halide cameras, Japanese Patent Publication No. Sho 58-53327 discloses the art of automating or simplifying the process of checking a negative film in a printing process, by using light-source decision information recorded on the negative film. Regarding a method for determining the kind of light source, an example using a color sensor is disclosed in Japanese Laid-Open Patent Application No. Hei 4-113339, or a proposal further having auxiliary means for detecting the flicker of a fluorescent lamp is also disclosed in U.S. Pat. No. 5,087,936.
In any of the conventional examples, a decision as to the kind of light source is made by using a color sensor made up of a plurality of sensors or by a combination of color detection and flicker detection.
However, the color sensor is comparatively expensive. The flicker detection is merely auxiliary means which makes a decision as to only a fluorescent lamp and needs to be used together with another means, thus leading to the problem that an expensive arrangement is needed.
As described above, the conventional methods of detecting the kind of light source are generally divided into two methods:
(a) a method of determining the kind of light source on the basis of the color of a light source; and
(b) a method of determining the kind of light source on the basis of the temporal variation (flicker) of the brightness of a light source.
FIGS. 20(A) to 20(C) show variations in the brightness of different kinds of artificial light sources.
As shown in FIG. 20(A), a fluorescent lamp emits flicker at a frequency twice as higher as a commercial power source frequency, and exhibits a waveform which is similar to a waveform obtained by half-wave rectification of the alternating current of a power source. The ratio of a maximum value to a minimum value of the amount of light emitted from the fluorescent lamp is normally greater than or equal to two.
As shown in FIG. 20(C), a fluorescent lamp which performs an inverter type of high-frequency lighting emits flicker at high frequencies of 40-50 kHz. The waveform of this type of fluorescent lamp often contains far higher-frequency components. In many of this type of fluorescent lamp, the ratio of a maximum value to a minimum value of the amount of light is normally approximately two, similarly to the normal type of fluorescent lamp.
As shown in FIG. 20(B), an incandescent lamp (tungsten lamp) emits flicker having a sinusoidal waveform the frequency of which is approximately twice as high as the commercial power source frequency. The ratio of a maximum value to a minimum value of the amount of light is 1.1 to 1.2, smaller than that obtainable with the fluorescent lamp.
FIG. 21 shows another illumination example. In this illumination example, to reduce flicker, a plurality of fluorescent lamps are lit with their lighting waveforms out of phase with each other, and illumination is effected by the resultant amount of light.
Although the illumination example shown in FIG. 21 exhibits a flicker characteristic different from that of any of the above-descried examples shown in FIGS. 20(A) to 20(C), there is no large difference in light-source color between the example shown in FIG. 21 and the example shown in FIG. 20(A) or 20(C). For this reason, it is necessary to regard them as an identical light source in terms of color reproduction.
Since the conventional methods of detecting the kind of light source require a dedicated sensor and circuit, an increase in cost is incurred and a space in which to dispose such a sensor is needed.
In the method (b) of determining the kind of light source on the basis of flicker, the light measuring sensor of a camera may also be utilized. However, in a method of determining the presence or absence of flicker on the basis of the amount by which the amount of light varies, for example, the ratio of a maximum value and a minimum value of the amount of the variation, it is difficult to discriminate between flicker and a noise signal if the amount of flicker is small as shown in FIG. 21. In addition, as far as a simple variation is employed as a criterion for making a decision as to the presence or absence of flicker, if the luminance of a subject varies, the light measuring sensor of the camera which measures the luminance of the subject may erroneously determine that sunlight is a flickering light source.
The method of determining the kind of light source by using the light measuring sensor of the camera has a further problem. In general, the light measuring sensor of the camera is originally designed to measure steady light, and is not necessarily suited to detect flicker. Particularly in a light measuring sensor using the logarithmic compression circuit shown in FIG. 22, no sufficient output is obtained in a high-frequency range owing to a parasitic capacity, with the result that it is difficult to accurately detect flicker in the case of a high-frequency lighting fluorescent lamp. Such a circuit has the problem that, as indicated by the frequency-output characteristic shown in FIG. 23, a high-frequency output greatly lowers in the case of the lower-side curve representative of a low subject luminance and a small amount of light current.